For example, the following schemes are given as related-art optical modulation schemes, which have been used in an optical communication system.    On-off keying (OOK)    Binary phase-shift keying (BPSK)
In recent years, in order to deal with an increasing amount of traffic flowing through the Internet, there has been studied a scheme, which utilizes a multilevel phase modulation signal using a digital signal processing technology, and is capable of achieving a large-capacity communication system. For example, the following methods are given as such a multilevel modulation optical transmission and reception method, which utilizes a multilevel phase modulation signal.    Quadrature phase-shift keying: (QPSK)    Differential QPSK (DQPSK)    Eight quadrature amplitude modulation (8QAM)
As compared with the related-art scheme, which allocates the intensity of an optical signal to a binary signal to directly perform detection, in a digital coherent reception scheme, the intensity of an optical signal and phase information are subjected to analog/digital (A/D) conversion, and the resultant is demodulated by a digital signal processing unit.
In the above-mentioned digital coherent reception scheme, there is an advantage in that a mechanism for synchronizing frequencies and phases of a transmission light source and a local oscillator light source with reception light can be implemented as digital signal processing. With this, without the need to use an optical phase-locked loop (PLL), which is difficult to be implemented, it is possible to synchronize the frequencies and phases of the transmission light source and the local transmission light source with the reception light.
On the other hand, as a representative framing scheme to be used for transmission of a client signal through an optical communication channel, there can be given a scheme, which uses an optical transport network (OTN) frame defined by the Internal Telecommunication Union Telecommunication Standardization Sector (ITU-T) (see, for example, Non Patent Literature 1).
With the scheme using the OTN frame, along with the progress in a wavelength multiplexing technology, it is possible to perform long-haul and large-capacity transmission with high reliability. Further, the scheme has such a function as centralization of monitoring and control, and hence the scheme is widely used mainly in a metropolitan network connecting metropolitan areas to each other and a core network connecting bases of Internet service providers or telecommunications carriers to each other.
Note that, in a multilevel modulation optical transceiver for transmitting the OTN frame by distributing the OTN frame to a plurality of lanes with the use of the flexibility in polarization/phase/wavelength of an optical signal, due to an influence of dispersion caused by the difference in polarization/phase/wavelength of the optical signal, skew between the respective lanes on the reception side becomes a problem in some cases.
The optical transport network-multi-lane distribution (OTN-MLD) is defined as a method for solving this problem. With the OTN-MLD, it is possible to correct the skew between the respective lanes by detecting a shift in a relative position of a frame synchronization pattern with the use of overhead information of the OTN frame.